1. Field of the Invention
The disclosed embodiments of the present invention relate to a carrier frequency offset (CFO) calibration method, and more particularly, to a CFO calibration method applied to an Orthogonal Frequency Division Multiplexing (OFDM) receiver.
2. Description of the Prior Art
In general, in an Orthogonal Frequency Division Multiplexing (OFDM) communication system such as an OFDM communication system 100 shown in FIG. 1, since there is a frequency difference between oscillators (not shown) in a transmission terminal 102 and a receiving terminal 104, causing to the carrier frequency offset (CFO) problem in frequency domain and sample clock offset (SCO) problem in time domain in receiving terminal 104.
The CFO and the SCO are main reasons of damaging the system carrier orthogonality. Under conditions of no calibration or lack of calibration, it will cause a severe Inter Carrier Interference (ICI). Thus, it is important about how to perform the CFO calibration in the OFDM receiver. A conventional method of solving the above problem is using a phase lock loop (PLL) to perform the CFO calibration. As shown in FIG. 2, a PLL 200 uses a phase detector (PD) 202 to find out a phase difference between an input signal and a local clock first, and then calculates a frequency difference by a math calculation. After passing through a loop filter 204, an output clock of a voltage-controlled oscillator (VCO) 206 will be adjusted according to the frequency difference in order to change the frequency of the local oscillator, to perform the CFO calibration or tracking. However, the PLL is usually realized in an analog way, and it is not only difficult in design, but also easy to be affected by process floating, and it requires more chip area.
In addition, there is another conventional method which can be realized by digital signal processing only. Specifically, this conventional method does not use the analog circuit or radio frequency (RF) circuit for the calibration, but simply performs the CFO estimation by a base band (BB) digital circuit, and performs a CFO compensation by signal processing to eliminate influences of the carrier interference. In general, besides the estimation and the compensation, it is practical to perform CFO tracking & SCO tracking based on a pilot signal in the signals, so as to reduce the influence caused by problems of inaccurate estimations and compensations. However, although it is easier to use the BB digital circuit to perform the CFO calibration, but the following problems might occur. First, a DC notch filter might be affected by the CFO and signals near DC are filtered and can not be restored. Second, when the CFO effect is serious, the CFO estimation capability of the digital circuit might reduce greatly due to preamble symbol detection errors or decode errors.
Thus, there is immediate requirement of an innovative CFO calibration method to reduce cost and error rate.